The present invention relates to internal combustion engines of the type comprising:
at least one intake valve and at least one exhaust valve for each cylinder, each provided with respective spring means for biassing the valve to a closed position, for controlling communication between respective intake and exhaust conduits and a combustion chamber, PA1 a camshaft for actuating the intake and exhaust valves of the engine cylinders by means of respective tappets, each intake valve and each exhaust valve being driven by a cam of said camshaft, PA1 wherein at least one of said tappets drives the respective intake or exhaust valve, against the action of said biassing spring means, with the interposition of hydraulic means including a chamber of fluid under pressure, PA1 said chamber of fluid under pressure being adapted to be connected through a solenoid valve to an outlet channel, in order to uncouple the valve from the respective tappet so as to cause quick closing of the valve, under the action of the respective biassing spring means, PA1 said hydraulic means further comprising a piston associated with the stem of the valve and slidably mounted within a guiding cylinder, said piston facing a variable volume chamber defined by said piston within said guiding cylinder, said variable volume chamber being in communication with the chamber of fluid under pressure through a connecting aperture formed in one end of said guiding cylinder, said piston having an end nose adapted to be introduced within said connecting aperture during the final portion of the piston travel when the valve is closed, in order to reduce the communication port between said variable volume chamber and said chamber of fluid under pressure, so as to brake the valve travel in proximity of its closed position.
An engine of the above indicated type is disclosed for example in European Patent application EP-A-0 803 642 of the same applicant.
The above described system provides a variable control of the opening of the intake and/or exhaust valves without altering the mechanical parts which control the valve displacement. In fact, whilst in a conventional valve driving system the movement of each intake or exhaust valve is only due to the geometry of the mechanical parts which drive the valve (cam, tappet, and rocker arm, if any), in the above described known system, the solenoid valve controlling the pressure chamber associated with a given valve can be driven open at any time this is desired (typically it is controlled by electronic control means depending upon one or more parameters of operation of the engine), so as to empty the above mentioned chamber of fluid under pressure (which usually is the lubricating oil of the engine) thus causing the quick closing of the intake or exhaust valve, under the action of the respective biassing spring means, even during a stage in which the respective cam would keep the valve open.
As already indicated above, the known solution provides for a piston associated with the valve which is slidably mounted within a guiding cylinder. The piston faces a chamber of variable volume defined thereby within the guiding cylinder and communicating with the chamber of fluid under pressure by means of a connecting aperture formed at one end of the guiding cylinder. In order to slow down the travel of the valve in proximity of its closed position, so as to avoid damages due to an impact at an excessive speed of the valve against its seat when the pressure chamber is emptied so as to uncouple the valve from the respective tappet, the above mentioned piston has an end nose adapted to be introduced within said connecting aperture during the final portion of the travel of the piston when the valve is closed, in order to reduce the communication port between the variable volume chamber and the chamber of fluid under pressure, thus braking the travel of the valve in proximity of its closed position.
Studies and tests conducted by the applicant have shown however that the braking effect thus obtained can become excessive if the fluid under pressure (typically the lubricating oil of the engine) has a high viscosity due to a low value of its temperature. Thus, for example, when the ambient temperature is low, for example in the order of -10.degree. C., and the engine has not reached a normal operating condition subsequent to a cold start, the viscosity of the oil may be such that the valve closing time becomes excessive. For example a lubricating oil which in normal operating conditions may have a kinematic viscosity in the order to 15 centistokes, may have a viscosity of up to 4000 centistokes at a temperature of -20.degree. C.